[The Note] Reactive dyes for digital textile printing: the difference between MCT and VS dyes

18 June 2020 | Jos Notermans

When it comes to reactive dyes in textile printing, the two dyes of choice are Monochlorotriazine (MCT) and Vinyl Sulfone (VS). But what differentiates these dyes from one another and how does that work out in digital printing? What makes one solution better suited to certain companies and printing processes than the other? 

To resolve common misconceptions about these dyes for once and for all, I will compare both dyes in this blog in terms of composition, deepness of black, sensitivity to processes and costs. In my explanation, I will refer to them as MCT and VS. To start with our comparison, let’s look at what MCT and VS are and what the main difference between both reactive dyes is.

The chemical difference between MCT and VS

Reactive dyeing is the most important method in coloring cellulosic fibres, like cotton or viscose. This makes the usage of reactive dyes very common in the digital textile printing industry. In the past, when dyes were first made, only vinyl sulfone was being used. Nowadays, the conventional side of printing has shifted mostly to MCT dyes, as they are less sensitive to variations in the process and show more vivid colors than the VS dyes. 

Before I explain why VS dyes were initially preferred for Black in digital textile printing, but companies now increasingly ask for MCT Deep Black, let’s look at what MCT dyes and VS dyes are and what differentiates them. To understand the difference between VS and MCT, you need to understand the fixation process of these reactive dyes. Reactive dyes are called reactive because they contain a chromophore that chemically reacts to the specific groups of the fibre, creating a true covalent bond. Due to this covalent bond between fibre and dye molecule the reactive dyes actually have excellent fastnesses.

The reactive group that is attached to the chromophore is what differentiates MCT dyes and VS dyes chemically. The chromophore is determining the color. In MCT dyes, the adhesion to the cellulose is established by the reactive monochlorotriazine group, while in in VS dyes the reactive group is vinyl sulfone. 

Mixture of colors versus pure black

VS dyes owe their intensity to the chemical composition of the colors. This is especially noticeable in the color black. Black VS dyes consist of a mix of maximum 2 dyes (majority navy blue with some orange), whereas a black MCT dye is by definition a mixture of multiple colors that create the color black together. Some ink retailers claim their black MCT dye is “pure black”, but by calling it pure, they imply that it consists of a black molecule. That is not possible with MCT dyes. What they often mean is that their MCT black is very neutral, where other MCT blacks tend to be more reddish (by preference of region or printer; see my blog about comparing colors in textile printing ink.

The fact that black MCT is a mixture of multiple colors and black VS has one dominant color and a supportive color, it results that a black VS dye is always purer and therefore perceived deeper than its MCT counterpart. On the other hand, MCT dyes provide printers with more flexibility in the printing process due to the fact that their finishing process is less critical. 

Being a mixture of multiple different colors has another consequence for MCT dyes. Colors are indicated by a CI-number (their Color Index). Not all CI-numbers are suitable for digital textile printing. Some CI-numbers can’t be applied used because of the size of the color molecules. Therefore, not every MCT mixture for conventional textile printing can also be used in digital textile printing.

The L-axis of MCT and VS

Especially the deepness of black is an important factor in fashion and thus an important decision factor when selecting reactive dyes for digital textile printing. LAB is an accurate way of measuring colors with three values: L, A and B. The color value “L” - which stands for “Lightness” - is a very important measurement for the color black. 

On the L-axis, 0 is black and 100 is white. Printing a L-value of 0 is still impossible in textile printing, but over time the industry managed to create deeper and deeper blacks. The table below shows the different black depths of our different MCT and VS dyes.

Schermafbeelding 2020-06-16 om 09.25.08
As you can see, for every black MCT dye there is a VS variant that is even deeper black. For example, the Black MCT Ultra HD is an amazing product with the deepest black MCT dye in the market, but the VS Ultra HD variant is still of better quality.

Steaming processes

The way a reactive dye fixates causes other differences between MCT dyes and VS dyes. For example, VS dyes are more susceptible to minor changes in the steaming process. If you steam the same printed fabric on three different units of the same steamer model, it may look different on the fabric steamed on one steamer in comparison to the other two fabrics printed on the other steamers. For example, one might look more brownish or even pinkish or another result is more bright than the others. Because of this challenge, demand for MCT dyes is increasing, as MCT dyes do not have the sensibility. However, the intensity of VS dyes is higher.

This critical steaming sensibility of VS dyes goes back to the way of steaming. In the early days, back when VAT dyes were used as a standard, all steamers had to be completely airtight. Oxygen was damaging to the process, which is why steamers had to be airtight or sealed. 

MCT dyes can be used in this standard process without any difficulties. However, if you want to use VS dyes, it is essential that some oxygen gets in during the steaming process. This is due to the fact that VS dyes are sensitive to hydrolysis which can happen if no air is let into the steamer. 

This would mean, that when you would want to use VS dyes because of their intensity, you would have to open up your steamer. Luckily, most dyes do not contain any VAT colorants anymore, which means it is no problem to “unseal” steamers without risking the quality of other dyes used.

 Schermafbeelding 2020-02-25 om 13.25.02

For experienced textile printers, it is often no difficulty to handle the more sensitive VS dyes. However, for new printing companies with less experience, it can be difficult to master the black VS dyes in the process. Also for companies that utilize multiple steamers or even locations to produce their quantities, it is desirable to rule out the sensitivity to any possible process fluctuation.

A steamer that is optimized for digital textile printing is able to let in air during the steaming. The newest steamers are able to automatically suck in the required air for the steaming process. Better isolation and more sustainability are often selling points of new (digital) steamers, but these factors have nothing to do with the digital process. Old steamers can also be adjusted in order to let in air, but convincing a printing company to open up a steamer that they were told to keep closed for 30 years can be a hard job.

The dilemma of reactive dyes

SPGPrints continues to research on ways to create a deeper black, and with each step in formulation that we achieve we make a VS and and MCT variation.

But it is always the case that VS dyes will always enable a deeper black ink than MCT dyes in a similar ink formulation. The downside of VS dyes is that they are harder to control within the printing and steaming process. This leads to a dilemma. Our customers want to print the deepest black possible, but they don’t want it to cause problems within the printing process. VS dyes create a deeper black, but are also more sensitive in the production process.

The financial aspect: what do reactive dyes for digital textile printing cost?

The dilemma between MCT dyes and VS dyes does also show in their pricing. Generally, MCT is more expensive than a VS dye, which applies to black as well as the colored dyes. 

The individual ingredients for VS inks are cheaper, which is why making the exact same tone with MCT inks is more expensive. That is partly because of the more expensive ingredients (dyes), but the purifying process is also a factor that increases the price. Purifying VS dyes is less time consuming than MCT dyes.

Because of these factors, we can create a black MCT dye that is blacker than any MCT dye on the market, but it will be more expensive to make than VS dyes. In return, this MCT is more stable and less susceptible to changes in the printing process.

Pros and cons: conclusion

Now that we've talked about composition, sensitivity, quality and price of reactive dyes in digital textile printing, we can strike a balance between MCT and VS dyes.

In short, VS dyes are more attractive in terms of pricing and color density, but you need to know exactly what you’re doing and you have to be in full control of your production process. In terms of sensitivity, MCT is the safer choice, but to create an equal quality, MCT is more expensive. If the MCT Ultra HD Black is deep enough, choosing for safety might be a wise choice in the end.

If you have any more questions or if you would like more information about the different types of dyes, please feel free to contact us. Our specialists are happy to inform you about the technical backgrounds of reactive dyes. With their knowledge, they can help you choose the right reactive dye for your printing process. 

As you could read, one of the things that differs between conventional and digital textile printing is the steaming process. Do you want to make the transition to digital textile printing? I recommend reading our Guide, which covers all the factors relevant to this switch:

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Categories: Digital Textile Printing, Rotary screen printing, Single-pass Printing, Archer technology, Digital Printing, Digital printing inks, Multi-pass Scanning Printing, JAVELIN, PIKE

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